Yes, captain is a common noun, a word for any captain of anything.
A proper noun is the name of a specific person, place, thing, or a title; for example:
The plural form of the noun captain is captains. The plural possessive form is captains'.
The possessive form for the plural noun wives is wives'.
The plural possessive noun of molecules is molecules' .
The plural form for the noun buzz is buzzes; the plural possessive form is buzzes'.
The plural possessive noun for "vowel" is "vowels'".
The word sisters is the plural form; the plural possessive is sisters'.
The plural form for the noun tree is trees.The plural possessive form is trees'.
The noun "branch" is a regular plural noun, a noun that forms its plural by adding "s" or "es". The plural form of branch is "branches".The plural possessive form is branches', a regular plural possessive.
pLURAL ; orbits Possessive plural ; Orbits' (NB note the position of the apostrophe).
The plural possessive noun is racers'.
The plural possessive form for the noun senators is senators'.
Singular; Grandchild Singular possessive ; Grandchild's Plural ; Grandchildren Plural possessive ; Grandchildren's